Some early work on efficient “fiber to the home” architectures was done in the 1990s by the Full Service Access Network (FSAN) working group, formed by major telecommunications service providers and system vendors. The International Telecommunications Union (ITU) did further work, and has since standardized two generations of passive optical networks. PON is a point-to-multipoint, “fiber to the premises” network architecture in which unpowered passive optical splitters may be used to enable a single optical fiber to serve multiple premises, typically 32 up to 128. A PON typically comprises an Optical Line Termination (OLT) at the service provider's central office and a number of Optical Network Units (ONUs) or Optical Network Terminals (ONT) near end users. A PON configuration typically reduces the amount of fiber and central office equipment required compared with point-to-point (PTP) architectures.
Downstream signals in PON are typically broadcast to each premise sharing a single feeder fiber. Upstream signals are typically combined using a Multiple Access Control (MAC) protocol based on Time Division Multiple Access (TDMA). The OLTs typically configure the served ONTs in order to provide time slot assignments for upstream communication.
Different variants of PON architectures have been specified. APON (ATM Passive Optical Network) was used primarily for business applications, and was based on ATM. BPON (Broadband PON) is a standard based on APON. It adds support for additional RF video services provided on a separate optical channel via WDM, dynamic and higher upstream bandwidth allocation, and survivability. It also creates a standard management interface, called OMCI, between the OLT and ONU/ONT, enabling mixed-vendor networks. GPON, which is an evolution of the BPON standard, supports higher rates, enhanced security, and the choice of the Layer 2 protocol (ATM, TDM and Ethernet via GEM). In addition, the IEEE issued an Ethernet PON (EPON or GEPON) standard in 2004, as part of the Ethernet First Mile project. EPON typically uses standard Ethernet frames with symmetric 1 gigabit per second upstream and downstream rates.
The ITU-T G.984 GPON standard represents a boost in both the total bandwidth and bandwidth efficiency through the use of large, variable-length packets. The standard permits several choices of bit rate, but the industry has converged on 2,488 megabits per second (Mbit/s) of downstream bandwidth, and 1,244 Mbit/s of upstream bandwidth. To allow such bandwidth over a distance of 20 km when using a 64-way optical splitter, a demanding optical budget of 28 dB is required.
GPON takes advantage of Wavelength Division Multiplexing (WDM), using one wavelength for downstream traffic and another for upstream traffic on a single standard single-mode fiber (ITU-T G.652). The specification calls for downstream traffic to be transmitted on the 1490 (±10) nanometer (nm) wavelength and upstream traffic to be transmitted at 1310 (±50) nm wavelength. The 1550 nm band is allocated for optional overlay services, typically RF video (in the range 1550-1560 nm). Furthermore, GPON is a shared network, in that the OLT sends a single stream of downstream traffic that is received by all ONTs. Each ONT typically only reads the content of those packets that are addressed to it. Encryption is typically used to prevent eavesdropping on downstream traffic.
In the following and in the claims, a wavelength of about 1310 nm is referred to by the rounded value of 1.3 μm and a wavelength of about 1490 nm wavelength is referred to by the rounded value of 1.5 μm for the sake of simplicity.
The present document relates to extending the reach of PON or WDM-PON systems using two or more different optical wavelengths for the downstream and the upstream direction. In more general terms, the present document relates to providing an extension of the optical budget in optical access networks. This budget extension should be achieved in a cost-effective manner. Furthermore, the budget extension should be transparent to the underlying optical signals.